Many exercise devices are marketed, for example treadmills and ski simulators. Some exercise devices are lightweight and small in size so they can be considered portable. Other exercise devices are large or heavy such that a user cannot easily transport them from place to place.
Typically an exercise device is used to assist in performing exercise, for example by making it more fun and/or by helping to train specific muscles or to perform a specific motion. Some devices are used for rehabilitation of a person with injury and others to strengthen specific muscles.
Some devices use an elongated track with a platform that can move back and forth across the track, for example a SRF Board as sold by FitterFirst (www.fitter1.com). Typically, the user forms physical contact with the platform and causes it to move back and forth across the track, for example by standing on the platform with one leg and placing the other leg on the floor or holding on to an external stationary support.
U.S. Pat. No. 5,342,264 to Gordon describes an aerobic exercise device that provides for a smooth, natural, orbital continuous motion of the user's feet. In Gordon there is provided two parallel tracks with platforms. The platforms are controlled similarly to enable a user to jog, run, walk and perform other coordinated motion.